For automobiles and respective machine parts, many surface hardening treated parts are used. The surface hardening treatment is performed with the aim of improving abrasion resistance and fatigue strength, and as a representative surface hardening treatment method, carburizing, nitriding, induction heating, and the like can be cited. Nitriding treatments such as gas nitriding, gas nitrocarburizing, and salt-bath nitrocarburizing are performed at a transformation point to austenite or lower unlike other methods, to thus need a treatment time for several hours but has an advantage of capable of making heat treatment strain small.
Thus, the nitriding is a surface hardening treatment suitable for high-precision worked parts such as a crankshaft and a transmission gear in terms of automobile members or members requiring product shape accuracy after a hardening treatment of a damper disc and a damper plate formed by being pressed. Regarding the nitriding treatment, gas nitrocarburizing, salt-bath nitrocarburizing, and so on can be cited, but gas nitriding to be performed in an ammonia atmosphere makes it possible to obtain high surface hardness but generally needs a treatment time of 20 hours or longer because diffusion of nitrogen is slow. On the other hand, a nitrocarburizing treatment to be performed in a bath or an atmosphere containing carbon together with nitrogen such as gas nitrocarburizing or salt-bath nitrocarburizing makes it possible to accelerate diffusion speed of nitrogen. As a result, the nitrocarburizing treatment makes it possible to obtain a part having an increased surface hardened layer depth for several hours. By such a nitriding treatment, it is possible to form a surface hardened layer having an increased surface hardening depth, suppress fatigue crack initiation in the surface of a part, and improve fatigue endurance.
For increasing the surface hardened layer depth and surface hardness, a steel containing nitride forming alloys has been proposed to be disclosed in Patent Document 1, for example. Further, regarding a part obtained by press forming a hot-rolled steel sheet or a cold-rolled steel sheet, a gas nitrocarburizing treated steel sheet having improved workability at the time of press forming before a nitriding treatment and having an improved part surface hardness property after the nitriding treatment has been proposed to be disclosed in Patent Documents 2 and 3, for example. In each of the previously described well-known documents, for the improvement of surface hardness by the gas nitrocarburizing treatment, elements such as Al, Cr, and V being nitride forming elements are effective to be contained as alloying elements of a steel sheet for gas nitrocarburizing.